Crystallization is a technique which chemists use to purify solid compounds. It is one of the fundamental procedures each chemist must master to become proficient in the laboratory. Crystallization is based on the principles of solubility: compounds (solutes) tend to be more soluble in hot liquids (solvents) than they are in cold liquids. If a saturated hot solution is allowed to cool, the solute is no longer soluble in the solvent and forms crystals of pure compound. Impurities are excluded from the growing crystals and the pure solid crystals can be separated from the dissolved impurities by filtration.
This simplified scientific description of crystallization does not give a realistic picture of how the process is accomplished in the laboratory. Rather, successful crystallization relies on a blend of science and art; its success depends more on experimentation, observation, imagination, and skill than on mathematical and physical predictions. Understanding the process of crystallization in itself will not make a student a master crystallizer; this understanding must be combined with laboratory practice to gain proficiency in this technique.
What Happens During a Crystallization
To crystallize an impure, solid compound, add just enough hot solvent to it to completely dissolve it. The flask then contains a hot solution, in which solute molecules - both the desired compound and impurities - move freely among the hot solvent molecules. As the solution cools, the solvent can no longer hold all of the solute molecules, and they begin to leave the solution and form solid crystals. During this cooling, each solute molecule in turn approaches a growing crystal and rests on the crystal surface. If the geometry of the molecule fits that of the crystal, it will be more likely to remain on the crystal than it is to go back into the solution. Therefore, each growing crystal consists of only one type of molecule, the solute. After the solution has come to room temperature, it is carefully set in an ice bath to complete the crystallization process. The chilled solution is then filtered to isolate the pure crystals and the crystals are rinsed with chilled solvent.
This first series of diagrams shows what happens if you let a crystallization proceed slowly: first by setting the flask at room temperature undisturbed until crystals form, and then carefully on ice. The red bar to the right of each image is a thermometer, to indicate the temperature. The yellow triangles are an impurity in the hot solution of orange hexagons. If the solution is allowed to cool slowly, the impurities may attach briefly to the growing crystal lattice, but they soon leave as a compound with a more suitable geometry comes in to take their place. Suitable hexagons stay more readily in the growing lattice, and eventually pure crystals of orange hexagons are formed.
This second series of diagrams shows what happens if you cool the solution too quickly. The yellow triangle impurities are trapped inside the crystals being formed by the orange hexagons, thus, the crystals isolated are impure. Note that slow crystallization gives larger crystals than fast crystallization. Small crystals have a large surface area to volume ratio and impurities are located on the surface of the crystals as well as trapped inside the matrix.
Watch the crystallization video: